After AT&T announced it would be ditching unlimited data plans, a lot of customers were upset. Luckily, the changes only apply to new customers, so if you have an unlimited data plan now, you're grandfathered in. But, since the new rules also apply to the relatively new iPad 3G models, some potential 3G owners were quite unhappy with the change. In response, AT&T announced that iPads purchased before June 7 will be able to sign up for the $30 unlimited data plan.
This rule applies to iPad order dates, not shipping dates, so even if your new iPad 3G isn't scheduled to arrive until after June 7, as long as you purchase it before that date you can opt for the unlimited plan.